Masu, your touch up problem is probably too much thinner & no primer.
Rub down the model before painting with a nylon pan scrubber & warm water. leave to dry thoroughly (next day) prime (I use Halfords car plastic primer, in a spray can, though I doubt you have a branch on Malta) again, the next day, start painting, thin the enamel with about 30 % white spirit/turps & paint from front to back. Now make suer you haven't missed any bits. Clean brushes & allow model to dry overnight, then do the next colour.
Painting from front to back will get you a slightly streaky finish, just like the real thing.
I did have an airbrush once, & was shocked at the cost of a replacement propellant can, so went back to brushes, I'm happy with it, & so are my customers.
Merry Christmas
Pete